DocBook/drm: Insert `it' for smooth reading

Signed-off-by: Michael Witten <mfwitten@gmail.com>

Documentation/DocBook/drm.tmpl

@@ -458,7 +458,7 @@
 	  core GEM initialization.  A UMA device usually has what is called a
 	  "stolen" memory region, which provides space for the initial
 	  framebuffer and large, contiguous memory regions required by the
-	  device.  This space is not typically managed by GEM, and must
+	  device.  This space is not typically managed by GEM, and it must
 	  be initialized separately into its own DRM MM object.
 	</para>
 	<para>